Ten patients with angina pectoris and fatal cardiac arrest during cardiac catheterization were analyzed to determine the circumstances of death and the severity and distribution of the coronary arterial narrowing. Nine patients died during attempted coronary angiography, and the remaining patient, during right-sided cardiac catheterization. All 10 patients had at least three of four major epicardial coronary arteries narrowed 76-100% in cross-sectional area by atherosclerotic plaques, including seven with this degree of narrowing of the left main coronary artery by plaque; two additional patients had severe (greater than 75%) narrowing of the left main coronary artery by thromboembolic material superimposed on small atherosclerotic plaques. Of 354 five-mm long segments of the left main, left anterior descending, left circumflex and right coronary arteries in eight patients, the percent narrowed to various degrees in cross-sectional area by athersosclerotic plaques was as follows: 96-100% = 9; 51-75% = 23; 26-50% = 13, and 0-25% = 6.